Originally Posted by LDB

What physically happens in your engine when you use 87 and floor it is that it starts to knock, the knock sensors sense that, and the ECM almost instantaneously retards the spark to stop the knock. Some power and mileage is lost, but the Vette does so well in those regards that it probably doesn’t matter much to power or mileage unless you are a racer or are competing in an economy run. In my mind the real risk is that the knock sensors, while good, aren’t necessarily perfect. If they don’t retard the spark in near-instantaneous fashion, you are in big trouble. And even if they do retard as designed, you have still suffered mild pinging for an instant, and that can’t possibly do any good to the engine. Still worse, you keep going through that cycle over and over and over and over and over again, because the ECM tries to get back to design spark advance, so it keeps on going through those sensing cycles. Pings detected, retard, pings stop, timing creeps back toward design, pings again, retard, over and over and over again. It’s utterly inconceivable that does your engine any good. The only question is how much harm it does. If the system runs flawlessly, there probably isn‘t much harm. But how many things run flawlessly for how long? In my view, the system is there for a backstop in case you can’t get premium fuel now and then. But I don’t think it’s intended as a system to allow continuous running on 87. If that was the intent, it would not have the logic built into it to keep trying to get back to design spark advance, and you would not have to keep going through the cycles described above over and over and over again.

As to the altitude impact, yup, at that altitude, you need a couple of octane less. But the grades at the gas stations are also usually a couple of octane less at stations in high altitude areas. So while regular/mid/premium at low altitudes may be 87/89/93, at altitude, they are probably something like 85/87/91. So to avoid the cycles described above, you still need to be using at least 91 at altitude, which may well be premium as sold in your area.

Originally Posted by PatternDayTrader

Here's a nifty quote in case you don't want to read it

"We expect octane demand on gasoline suppliers in high-altitude areas to increase as these new cars make up a larger part of the vehicle population, and this could raise the cost of gasoline"

Originally Posted by JoesC5

Sorry, but you are the one who is wrong. Why is regular only 85 octane at the gas station pumps in Denver, etc?

As for the OP. he shouldn't use less than 89 octane at 5,000' in a car that requires a minimum of 91 at low altitudes.

When I'm in Colorado in my C6 Z06, I do use 91 as I'm not trying to save a few pennies and 93 here at home(1320' altitude).

Originally Posted by JoesC5

What they are saying is because more new cars require Premium gas(91+) then previously, then gasoline suppliers will see an increase in demand for higher octane gas(91+) by the public. That additional demand for premium gas could then result in shortages, thus causing price increases, plus the wholesale cost of premium is higher than the wholesale price of regular..

One way the car manufacturers are increasing their CAFE is by raising the engine's compression ratio, thus increasing the cars efficiency(and gas mileage), and also more new cars are adding forced induction to a small displacement engine, and the use of a turbocharger/supercharger normally necessitates the use of premium gas.

Originally Posted by COCorGS

Okay so I only have about 2400 miles on my '19 GS that I just got at the end of August. No performance mods yet, bone stock, except for a Range AFM in the ODB II port to keep it in 8 Cyl. mode. For the first time I tried 87 octane to see what effect, if any, it had on the car. The salesperson at the dealer told me premium fuel was required, but I later read in the owner's manual that 87 will work. It said there may be some knocking and to use only premium if that occurs. It didn't. I figured it wouldn’t because it was the same with my '15 Camaro. There is a sensor in both models that tells the ECM to change the timing based on what fuel you put in it. The Camaro had an issue switching back and forth from 87 to 91 and losing performance and the forums talked about a fuse pull to reset the tables to 91. Hopefully the Vette doesn’t have the same problem. I'm aware that it may effect both mileage and horsepower if I put in the lower octane fuel, at least that's what the owner's manual says. And who wants lower HP? Certainly not me but I wanted to see if the butt dyno could feel it and if the mpg’s would go down.

Well so far I've only driven it about 50 miles, this morning, under both WOT and cruising at 65 using the instant mileage calculator gauge, and I'd have to say it's not that much of a difference. Said the guy who measures his car in tenths, LOL. I'm sure lap times would have shown a difference, but I experienced zero knocking or any kind of technical problems. It didn't really matter in daily driving. I was using the horsepower gauge in WOT though and it never went above about 380. That was up to 4th gear at close to redline. Unfortunately I'm not sure if I ever got it to 460 HP with premium fuel in it because I don’t remember and I hadn't looked at it in a while. I will say that where I usually drive is back country roads in the foothills of the Rockies and it's around 6600 feet high in elevation so that has to be effecting the HP. I read somewhere that your car loses about 3% of HP per 1000 ft above sea level, which means I would have already lost 91 hp where I drive. Here's the formula I found if anyone is interested: HP loss = Elevation x .03 x HP @ Sea Level / 1,000.
My instant fuel gauge was showing about 35 mpg cruising 65 mph in 8th gear on relatively flat roads at about 1500 rpm. I tried this several times for varying distances and this was the average number I kept getting. I know it's not a full tank over a long span but that's the scope of my research so far.
So the question is who else has experimented with different octanes and come up with their own results? And who out there has done some mods and seen some differences on the horsepower gauge, not just the dyno results? It'd be cool to hear about someone adding long tubes, an exhaust maybe with cat delete, a CAI and a tune and getting the gauge to the 460 mark really easily, like it wanted to keep right on going. I think it would be cool if you could use that gauge instead of having to dyno the car to find out if the mods effected the HP. Hopefully this topic hasn't already been exhausted, I tried to search and didn't find anything.

Originally Posted by LDB

I agree with JoesC5 that the pumps must state octane, and the octane stated there is correct. My point was that a person not familiar with the octane/altitude issue may not be aware that the various grades are posted as lower octane as altitude increases. So you just have to look at what’s posted in comparison to your needs

Not sure what PatternDay’s point is. He apparently thinks there isn’t an altitude effect, yet his own link says there is. As far as pushing down harder on the gas, sure, that works up to a point. The point where it no longer works is when the high altitude car’s pedal is to the floor to keep up with the sea level car at part throttle. The reason you need less octane at high altitude is that octane need is greatest when power is at max. Since the high altitude car can never get to max sea level power due to the air being thinner, it doesn’t need as much octane. It would be like taking a sea level car and putting a ping pong ball under its gas pedal so it could never be floored. If you couldn’t floor it at sea level, you wouldn’t need as much octane at sea level either. Just like an athlete can’t get as much oxygen in Denver so can’t run as far or as fast in Denver as he can in Houston, a car can’t get as much oxygen either, so can’t develop as much power or need as much octane.

Originally Posted by LDB

I agree with JoesC5 that the pumps must state octane, and the octane stated there is correct. My point was that a person not familiar with the octane/altitude issue may not be aware that the various grades are posted as lower octane as altitude increases. So you just have to look at what’s posted in comparison to your needs

Not sure what PatternDay’s point is. He apparently thinks there isn’t an altitude effect, yet his own link says there is. As far as pushing down harder on the gas, sure, that works up to a point. The point where it no longer works is when the high altitude car’s pedal is to the floor to keep up with the sea level car at part throttle. The reason you need less octane at high altitude is that octane need is greatest when power is at max. Since the high altitude car can never get to max sea level power due to the air being thinner, it doesn’t need as much octane. It would be like taking a sea level car and putting a ping pong ball under its gas pedal so it could never be floored. If you couldn’t floor it at sea level, you wouldn’t need as much octane at sea level either. Just like an athlete can’t get as much oxygen in Denver so can’t run as far or as fast in Denver as he can in Houston, a car can’t get as much oxygen either, so can’t develop as much power or need as much octane.

Originally Posted by ptalar

I own other cars that use 87. Once in a while when I fill up at Costco I forget that I am driving the Vette and I put regular in. Not intentionally. When you have 4 cars and 3 run on 87 it is easy to forget once in a while. But I don't race my Vette so I am not too concerned if I put some 87 in every now and then.

Originally Posted by LDB

So while regular/mid/premium at low altitudes may be 87/89/93, at altitude, they are probably something like 85/87/91.